There is available in the market today glass substrates, such as large glass windows, which have a tin oxide coating on a selected surface thereof. The purpose of the tin oxide coating is to improve the emissivity value of the window over that emissivity value which it would have without a coating thereon. Normally, the emissivity value of a glass substrate, such as a window, is improved by the application of a tin oxide coating thereto. Generally it may be said that a body having an emissivity value of 1.0 would pass all incident infrared radiation therethrough while a body having an emissivity value of 0 would reflect all incident infrared radiation. A clear glass window having a thickness of 1/8 inch has an emissivity value of 0.84, whereas such a glass window coated with a tin oxide coating would have an emissivity value in a range of 0.50-0.35.
The lower the emissivity value, the better the coated glass substrate is in reflecting infrared radiation. For example, if such a coated glass substrate is glazed into a window, the coating is effective in reflecting back into a building the infrared radiation produced within the building as, for example, by means of a fuel burning furnace. Most of such infrared radiation would normally pass through an uncoated window, but will have a large proportion thereof reflected back into the building by a properly coated glass window.
This disclosure teaches the selection of a particular heat decomposable, organic, tin-containing material for providing the tin which is oxidized to form a tin oxide coating on a selected surface of a glass substrate. The disclosure also teaches the use of a particular organic acid to provide a dopant for the tin oxide film which is being formed on the selected surface of the glass substrate. My invention is the discovery that if the ratio of the tin-supplying material to the dopant material is controlled within certain ratios, and the temperature of the selected surface of the glass substrate is controlled at a temperature within a certain temperature range, the best emissivity characteristics will be obtained from the doped tin oxide coating on the selected surface of the glass substrate that can be achieved by a reaction of the particular reactant materials I have selected.
The organic, heat decomposable, tin-containing material that I have selected is dibutyltin dibutoxide and the doping material that I have selected is trifluoroacetic acid. These materials have been selected as they form a very clean coating of doped tin oxide on a heated glass substrate by a pyrolytic decomposition process. In such a pyrolytic process, a tin oxide coating is developed on a selected surface of a glass substrate by application of a coating material to a heated glass substrate. Selected chemical materials can be sprayed in a dissolved form against the selected surface of the heated glass substrate in an oxidizing atmosphere. The so-sprayed materials coming into the vicinity of and/or contact with the selected surface of the glass substrate will be heat decomposed permitting the tin contained in the tin-containing material and the dopant therefor to be oxidized and deposited as a doped tin oxide coating on the selected surface of the glass substrate.
It is the primary object of this invention to provide a method of achieving the best emissivity characteristics from the reaction of certain materials used to produce doped tin oxide coatings on selected surfaces of glass substrates. The particular reactant materials which I have selected have not been commercially used as far as I am aware for the purpose of achieving doped tin oxide coatings on glass substrates.
I have filed on even date herewith an application entitled "Glass Substrate Coated with Tin Oxide and Method of Making Same", which has been assigned Ser. No. 083,479. This application is hereby incorporated by reference.